Hash table and linked list implementation of the Set interface,
with predictable iteration order. This implementation differs from
HashSet in that it maintains a doubly-linked list running through
all of its entries. This linked list defines the iteration ordering,
which is the order in which elements were inserted into the set
(insertion-order). Note that insertion order is not affected
if an element is re-inserted into the set. (An element e
is reinserted into a set s if s.add(e) is invoked when
s.contains(e) would return true immediately prior to
the invocation.)

This implementation spares its clients from the unspecified, generally
chaotic ordering provided by HashSet, without incurring the
increased cost associated with TreeSet. It can be used to
produce a copy of a set that has the same order as the original, regardless
of the original set's implementation:

void foo(Set s) {
Set copy = new LinkedHashSet(s);
...
}

This technique is particularly useful if a module takes a set on input,
copies it, and later returns results whose order is determined by that of
the copy. (Clients generally appreciate having things returned in the same
order they were presented.)

This class provides all of the optional Set operations, and
permits null elements. Like HashSet, it provides constant-time
performance for the basic operations (add, contains and
remove), assuming the hash function disperses elements
properly among the buckets. Performance is likely to be just slightly
below that of HashSet, due to the added expense of maintaining the
linked list, with one exception: Iteration over a LinkedHashSet
requires time proportional to the size of the set, regardless of
its capacity. Iteration over a HashSet is likely to be more
expensive, requiring time proportional to its capacity.

A linked hash set has two parameters that affect its performance:
initial capacity and load factor. They are defined precisely
as for HashSet. Note, however, that the penalty for choosing an
excessively high value for initial capacity is less severe for this class
than for HashSet, as iteration times for this class are unaffected
by capacity.

Note that this implementation is not synchronized.
If multiple threads access a linked hash set concurrently, and at least
one of the threads modifies the set, it must be synchronized
externally. This is typically accomplished by synchronizing on some
object that naturally encapsulates the set.
If no such object exists, the set should be "wrapped" using the
Collections.synchronizedSet
method. This is best done at creation time, to prevent accidental
unsynchronized access to the set:

Set s = Collections.synchronizedSet(new LinkedHashSet(...));

The iterators returned by this class's iterator method are
fail-fast: if the set is modified at any time after the iterator
is created, in any way except through the iterator's own remove
method, the iterator will throw a ConcurrentModificationException.
Thus, in the face of concurrent modification, the iterator fails quickly
and cleanly, rather than risking arbitrary, non-deterministic behavior at
an undetermined time in the future.

Note that the fail-fast behavior of an iterator cannot be guaranteed
as it is, generally speaking, impossible to make any hard guarantees in the
presence of unsynchronized concurrent modification. Fail-fast iterators
throw ConcurrentModificationException on a best-effort basis.
Therefore, it would be wrong to write a program that depended on this
exception for its correctness: the fail-fast behavior of iterators
should be used only to detect bugs.

LinkedHashSet

LinkedHashSet

Constructs a new linked hash set with the same elements as the
specified collection. The linked hash set is created with an initial
capacity sufficient to hold the elements in the specified collection
and the default load factor (0.75).